This invention relates to methods and apparatus for stabilizing the frequency of continous wave dye lasers and continous wave ring dye lasers.
Commercially available dye lasers can produce a line width in the order of 10 megahertz and a drift in the order of 10 megahertz per minute. However, this is not good enough for a great deal of spectroscopic work; and more narrow line widths are often required.
In theory, a dye laser operating in a single mode should produce a pure sine wave with an infinitesimal line width at the particular optical frequency of operation. However, in actual operation the line width is determined by a number of things including, mechanical jitter of the laser, variations in the optical path caused by acoustical waves, variations in the optical path caused by the dye jets in the dye laser, and other factors. Thus, while it has been desired to control the line width to 1 megahertz or less, achieving such narrow line widths has been difficult to do.
In attempts to achieve small line widths the prior art has, in general, tried to servo control the dye laser head by using a reference which is more stable than the dye laser head. A reference Fabry-Perot interferometer has been used for this purpose. In prior art systems of this kind a small part of the light output of the dye laser is directed through the interferometer to provide an output signal which changes in amplitude with a change in frequency of the dye laser. The reference interferometer thus changes frequency modulation into amplitude modulation. The amplitude change can be detected photoelectrically and has been used as part of a servomechanism to control the output frequency of the laser. The output frequency of the dye laser can thus be made characteristic of the reference interferometer rather than the laser head of the dye laser.
The goal then, with the reference interferometer control of this kind, has been to have the frequency stability of the laser head characteristic of the reference interferometer, because the interferometer usually can be smaller, more compact and more effectively environmentally sealed than the laser head itself, principally because of the smaller size of the reference interferometer.
There are two features of a reference interferometer that usually characterize the frequency stability of a servo system of this kind.
These features are the finesse and the free spectral range of the reference interferometer.
The free spectral range is generally determined by the length of the interferometer and gives the equivalent frequency difference between successive passbands of the interferometer.
The finesse is used as a measure of the resolution of a Fabry-Perot interferometer and is equal to the ratio of the separation between peaks to the width of a transmission bandpass. The width of a bandpass is measured at one half the maximum amplitude. The actual shape of the transmission curve of a Fabray-Perot interferometer is given by what is known as the Airy function, and the finesse is dependent in large measure on the reflectivity of the mirrors used. The finesse increases as the mirror reflectivity increases.
To stabilize the dye laser head to the reference interferometer one servo locks to a side of a transmission fringe of the reference interferometer with a zero for the error signal usually located approximately halfway up the fringe. Either the reference interferometer is tuned or the dye laser is tuned until the lock on the side of the fringe is made in this area. Thereafter a small change in frequency in the dye laser leads to a relatively large change in the amplitude transmitted through the reference interferometer.
To get the narrowest line width it is generally desired to use the smallest free spectral range for the reference interferometer because the line width that can be achieved by locking is determined in part by the free spectral range of the interferometer. A smaller free spectral range of the reference interferometer will enable the dye laser to produce a narrower line.
However, as the free spectral range of the reference interferometer is reduced, the system becomes less stable with respect to locking to successive passbands of the reference interferometer. That is, because the dye laser is tunable and can oscillate in a large number of possible modes, reducing the free spectral range of the reference interferometer increases the possibility that the dye laser can mode hop to one of the other permitted frequencies of oscillation and that the new frequency can lock on a side of a different transmission fringe of the reference interferometer without causing a noticeable change in the amplitude of the output signal of the reference interferometer.
Thus, using a reference interferometer with smaller free spectral ranges in order to stabilize the dye laser at a narrower line width actually increases the chances for an undetected mode hop. Attempting to provide a more stable reference can result in the reference becoming less stable.
The reason that a dye laser can hop like this is due to perturbations. These normally arise due to bubbles in the jet stream of the dye laser, but they can arise from other causes also.
In the event of a perturbation, the laser blinks out momentarily and comes back on. It may come back on at the same frequency or it may come back on at a different cavity resonance frequency.